FIG. 1 illustrates a subframe structure in which data and control information to be transmitted on a data channel are multiplexed and mapped to the data channel. A frame transmitted during one Transmission Time Interval (TTI) includes N×M Resource Elements (REs) that may be represented as a combination of N subcarriers and M Single Carrier-Frequency Division Multiple Access (SC-FDMA) symbols. Data and control information may reside in the REs on a modulation symbol basis.
In the frame, neither the data nor the control information is positioned in K REs having a Reference Signal (RS) and a Sounding RS (SRS). Therefore, the data may be carried in (N×M)-K REs. The data and the control information may have different modulation orders according to a transmission condition, a plurality of bits may be mapped to one symbol according to a modulation order, and one symbol is mapped to one RE. First, the amount of data and control information that can be delivered per SC-FDMA symbol is calculated. Then multiplexed data and control information are mapped sequentially to Resource Block (RB) 0 to RB (N−1) along a time axis (i.e. along an SC-FDMA symbol direction) on a subcarrier-by-subcarrier basis.
In FIG. 1, the control information may include first control information (control information 1), second control information (control information 2), and third control information (control information 3) or part of them. The multiplexed data and control information are mapped to the data channel on a modulation symbol basis according to a modulation scheme. The mapping proceeds to the right, starting from an uppermost left position of a first RB. In the same manner, modulation symbols are mapped to one subcarrier after another subcarrier.
Hence, the data and the control information are multiplexed through rate matching or puncturing to insert the control information between the data. The data and control information are not provided at the positions of the RSs and the SRS. Control information 1 is mapped along the SC-FDMA symbol direction, starting from an uppermost left RE of a subframe. Mapping of control information 2 starts with a last subcarrier, proceeding toward a first subcarrier, subcarrier 0. Control information 3 is mapped to REs each apart from the RS by one RE, in the direction from the last subcarrier toward subcarrier 0. The Data is eventually filled in the remaining REs from the control information mapping in a similar manner to the mapping of control information 1.
Control information 1 may be a Channel Quality Information/Precoding Matrix Index (CQI/PMI) being a combination of a CQI and a PMI. As its appellation implies, the CQI is information indicative of a channel quality, and the PMI is the index of a codebook used for precoding. Control information 1 may be multiplexed with the data by rate matching.
Control information 2 may be a Hybrid Automatic Repeat reQuest (HARQ) response, ACKnowledgment/Negative ACKnowledgment (ACK/NACK). Control information 2 may be multiplexed by puncturing the data or control information 1.
Control information 3 may be a Rank Indication (RI) indicating the number of transport streams. Control information 3 may be multiplexed by puncturing the data or control information 1 or rate-matching with the data and/or control information 1.
Puncturing is the process of eliminating predetermined bits (or symbols) in a bit (or symbol) sequence and inserting new bits (or symbols) at the empty positions. That is, puncturing amounts to replacement of part of information with another piece of information. When data or control information is multiplexed, information to be inserted substitutes for punctured bits (or symbols) of information. Despite the insertion of new information by puncturing, a total bit (or symbol) length is maintained unchanged. Yet, the puncturing affects the coding rate of the punctured information.
Rate matching is the process of adjusting the coding rate of data. When data or control information is multiplexed by rate matching, the position of each piece of information may be changed but the rate matching does not affect bits (or symbols) prior to multiplexing. That is, ‘rate matching’ of control information 1 and the data means that control information 1 and the data are processed such that their sum is a predetermined value. Accordingly, if control information 1 increases in amount, the amount of data to be rate-matched with the control information is reduced as much.
FIG. 2 illustrates a multi-carrier. In FIG. 2, the multi-carrier represents a total frequency band available to a Base Station (BS), equivalent to a whole band in its meaning.
A Component Carrier (CC) is an element of the multi-carrier. That is, a plurality of CCs form the multi-carrier by carrier aggregation. A CC includes a plurality of lower bands. If the multi-carrier is called a whole band, a CC may be referred to as a subband and a lower band as a partial band. Carrier aggregation is also called bandwidth aggregation.
Carrier aggregation refers to extending a bandwidth by aggregating a plurality of carriers in order to increase data rate. For example, 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) uses a carrier of 20 MHz and Long Term Evolution-Advanced (LTE-A) extends the bandwidth of the carrier up to 100 MHz by aggregating five 20-MHz carriers. Carrier aggregation covers aggregating carriers in different frequency bands.
The extended bandwidth for communications increases the transmission capacity of a transmission system, thus increasing control information associated with the transmission in amount. Also, when bandwidth aggregation is adopted for compatibility with legacy systems, control information is created for each band. As a result, more control information as well as more data are produced. Simple extension of conventional methods to transmit the increased control information may cause the following problems.
Firstly, control information 1 as well as the data is punctured to ensure the performance of control information 2 that has been increased in amount. The puncturing brings about the performance degradation of control information 1. Secondly, if the transmission bandwidth of the control information is extended to ensure the performance of all increased control information, a last Code Block (CB) of the data is concentratedly punctured, thereby degrading the performance of the data. Thirdly, when a conventional method is used for a band to which bandwidth aggregation is applied, increased control information concentrates on a specific band among aggregated bands. Thus, the above first problem may be generated.
Meanwhile, if a BS fails to decode control information 3 (e.g. an RI) received from a mobile terminal, it may not find the start of the data because it does not locate control information 1 (e.g. a CQI/PMI) accurately. Consequently, data decoding is affected. Especially in the case where the data is constructed in a plurality of CBS, decoding errors become serious. In case of an extended Cyclic Prefix (CP), if the SRS is included in a last SC-FDMA symbol, power transition distorts the symbols of control information 3 adjacent to the SRS. Thus, the overall performance of control information 3 may be degraded. Moreover, conventional techniques do not ensure compatibility with a transmission system that extends a bandwidth by aggregating a plurality of groups of carriers (e.g. LTE-A), due to an increase in bandwidth for communications.
FIG. 3 illustrates a subframe structure in which control information except data is mapped to a data channel. Referring to FIG. 3, if specific control information is mapped to symbols near to an RS to guarantee the performance of the specific control information, meaningless data such as blanks illustrated in FIG. 3 are created, thus increasing an occupied bandwidth. In other words, more subcarriers are unused and thus the communication capacities of other mobile terminals may be decreased as much as the number of the unused subcarriers. Therefore, there exists a need for a method for avoiding HARQ buffer corruption in an HARQ transmission scheme, while reducing a bandwidth as much as possible.